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Monday, March 23, 2015

In 1972 and 1973, NASA launched two simple spacecraft, Pioneer 10 and
11, to Jupiter. They were charged with
scouting the way for the more sophisticated spacecraft that would follow. Since then, the outer solar system has been
the realm of large, expensive missions: the Voyagers, Galileo, and Cassini.

Large missions costing well over $1B have proven very successful, but
they are launched at the rate of approximately one per decade and outer solar
system destinations have had to vie with Mars for these rare slots.

At last month’s Outer Planet Analysis Group, scientists and engineers
presented three proposals in competition for selection in NASA’s cheapest
category of planetary missions, the Discovery program. One other proposal may eventually vie for a
slot in NASA’s mid-range New Frontiers program.

The road to lower costs outer planet missions has been paved by NASA’s
first two New Frontiers missions, the $700M New Horizons mission en route to Pluto and the $1.1B Juno
mission en route to Jupiter. But can the cost of a mission to the outer
solar system be cut to $450M, the limit for a Discovery mission?

The three Discovery proposals take very different approaches.

The Enceladus Life Finder (ELF) team proposes to take the tightly
focused, minimalistic path.

We now suspect that many of the icy worlds in the outer solar system
harbor oceans beneath their icy crusts. Among
those bodies, Europa and Enceladus are special because their oceans appear to
rest atop their rocky cores, providing access to elements and minerals believed
essential to life. These rocky surfaces
are also are believed to have hot hydrothermal springs that could provide the
energy needed for the complex chemistry needed to support life. For other icy worlds, the oceans are
sandwiched between layers of ice and not in contact with their rocky cores,
making Europa and Enceladus priorities for exploring potentially habitable
worlds.

Enceladus so far is unique in having plumes of water that steadily jet
from its surface, spilling the contents of its ocean into space where they can
be easily sampled by a passing spacecraft.
(Hubble Telescope observations suggest that Europa may also have plumes,
but repeat observations have failed to confirm the initial sighting. Any plumes may be episodic, repeating only
every few years.)

The Cassini spacecraft currently at Saturn has already sampled the
plumes, but its instruments were designed in the 1990s, and weren't designed to
study the highly complex molecules that could indicate life. The ELF spacecraft would narrowly focus on
sampling these plumes with modern, highly sensitive instruments.

The mission would enter orbit around Saturn and then make ten flights
through Enceladus’ plumes. ELF’s instrument payload would consist of two mass spectrometers that
“weigh” atoms and molecules to measure composition.The spectrometers would analyze particles in
the plume that are a mixture of frozen ocean water and particles from the
seafloor. One of the mass spectrometers would be optimized to study the liquids
that originate from the ocean and the other the solid particles that likely
originate from the rocky core.A third
instrument might be included that would test whether any amino acids found have
predominately left- or right-handed structures.(Life on Earth predominately creates left-handed forms, and it’s
suspected that life that originates elsewhere will similarly favor one form
over the other instead of a random mixture likely from abiotic chemistry.)The spacecraft’s navigation camera also would
image the plumes to judge their activity at the time of each flyby.

To appreciate the simplicity of the ELF proposal, you need to consider
previous concepts for exploring Enceladus following Cassini’s last flyby late
this year. When Decadal Survey considered
this decade’s priorities for exploring the solar system, several Enceladus options
were considered. The simplest would have
been a multi-flyby spacecraft like ELF, but that would also have included two
cameras and an ice penetrating radar in addition to the mass
spectrometers. The preferred mission
would have studied Enceladus both during flybys and from orbit about the moon
with a somewhat different, but equally rich instrument compliment. These concepts would have generated volumes
of data that would have investigated the structure of Enceladus, its ocean, and
ice shell along with the chemistry of its internal ocean. Unfortunately, the estimated costs of these
missions were over $1.5B.

(Quoted mission costs often vary based on the specifics of what’s
included; in this post, I’ve tried to quote the costs for the items included
the widely-quoted $450M cap for Discovery missions. NASA’s final cost for a full mission
including launch and operations would be higher than the costs I’m including
here.)

The Io Volcano Observer would take a different tact than the ELF
mission. Like the proposed Enceladus
mission, the IVO spacecraft would observe Jupiter’s volcano-rich moon during
several flybys. Unlike the Enceladus
mission, the Io mission would carry four instruments – a two camera suite, a
thermal imager, a magnetometer, a mass spectrometer – along with a
student-built instrument to map volcanic hot spots. The spacecraft’s radio system would do double
duty by also allow precise tracking of the spacecraft’s speed during flybys to
study the distribution of matter within this moon. This would be a data rich mission, and the
spacecraft would carry an experimental high data rate optical communications
system in addition to the traditional radio system.

While ELF would focus on one investigation – the chemistry of the ocean
– IVO would perform an integrated series of studies to understand Io as a world
and a member of the Jovian system. The
mission’s goals are divided between understanding the sources and extent of its
intense volcanic activity, the effects of the injection of its volcanic plume
material into the wider Jovian system, and long term monitoring of Europa for
plumes and Jupiter’s atmosphere. The
nominal mission would last 22 months, but an extended mission might carry on
for an additional 6 years to monitor Io through time. If the extended mission occurred, then we
might have three spacecraft simultaneously studying the Jovian system in the
early 2030’s: IVO in a polar Jovian orbit with periodic flybys of Io, Europe’s
JUICE mission with broad studies of the Jovian system and a focus on the icy
moon Ganymede, and NASA’s Europa mission.

The third Discovery proposal takes an entirely different approach to
exploring the outer solar system on a budget.
The Kuiper mission would launch a space telescope dedicated to studying
the outer solar system.

The Kuiper proposal addresses two problems. First, we cannot afford to have spacecraft at
each of the major outer planets to observe their weather, their magnetospheres,
and their moons. This dedicated outer
solar telescope would be able to examine each of these worlds multiple times
each day to study these worlds as dynamic systems. Scientists will be able to observe how storm
systems in their atmospheres exchange energy, how variations in auroral
activity provides clues to the state of their magnetospheres, and how volcanic
and plume activities on the moons Io, Europa, and Enceladus vary over time.

The second problem is that we don’t understand key questions about the
formation of the outer solar system. We
suspect that the outer planets migrated during the early ages of the solar
system, but there are competing theories as to whether that migration was
smooth or more chaotic. As the planets’ orbits
shifted, they would have flung smaller bodies about. The Kuiper telescope would analyze the
spectra of thousands of small bodies ranging from Jupiter’s orbit to the
distant Kuiper belt to analyze their compositions. The mixture of compositions at different
distances from the sun would allow astronomers to distinguish between the
competing theories.

Telescopic observations have always played a crucial role in studying
the outer solar system. Earth-based
telescopes, however, have key limitations – any solar system target is visible
for only a few hours each day and our atmosphere blurs vision and blocks key
wavelengths of light. The science
proposed for the Kuiper mission could be done by the Hubble Space Telescope,
but its observing time is precious and little is allocated to solar system
studies.

The Kuiper mission would be smaller than Hubble (a 1.2 meter primary
mirror versus the Hubble’s 2.4 meter mirror) but would be dedicated to
observing the outer solar system. The
spacecraft would be parked in an orbit around the L2 Lagrange point beyond
the moon where it could observe the sky without Earth occultations and would be
beyond stray Earth light.

In addition to the three Discovery mission proposals, a fourth mission
concept was proposed, the LIFE Enceladus Sample Return.A previous incarnation of this mission was
proposed for the last Discovery competition but wasn't selected.Like the ELF mission, the LIFE mission would
make multiple flybys through Enceladus’ plumes and would use a mass
spectrometer to study their chemistry.Unlike
ELF, the LIFE spacecraft would collect dust plumes in a fashion similar to that
done by the Stardust spacecraft that collected comet dust samples in the
mid-2000s.The samples would later be
returned to Earth where far more sensitive measurements would be made than
could ever be done by instruments on a spacecraft.The cost estimates presented by the LIFE team
puts the mission outside the current scope of the Discovery program, and the
team is building support to add an Enceladus sample return mission to the New
Frontiers candidate mission list for the 2020s.

After 2017, there are no plans to have a spacecraft operating in the
outer solar system until the late 2020s at the earliest. That decade gap will exist because outer
planet missions in the past have had to be infrequent because of their
costs. The four proposals presented at
last month’s meeting represent the planetary community’s attempt to find a new
class of much lower cost missions that could fly more frequently. The Kuiper telescope would be an entirely new
approach to the problem that could begin providing data in the early
2020s. The missions to Io and Enceladus
face a tougher challenge because they propose to do missions for half the cost
of any previous outer planet mission orbiter.

Missions to both Enceladus and Io have been studied before, and the
costs were two to four times that of the cost cap for Discovery missions
($450M). Teams that propose missions are
generally fairly open about the great science their missions would do if they
are selected to fly. These teams tend to
be much more reluctant, however, to discuss the specifics of how they would
accomplish their goals within the tight cost caps of a Discovery competition –
that is their secret sauce. The teams
proposing ELF and IVO are seasoned veterans and their credibility gives me hope
that the outer solar system may open to low cost missions. We can assume that they have had a laser
focus on finding ways to reduce costs to a fraction of what previous studies
have assumed. Estimating development
cost, however, is always part art. NASA will
perform its own assessment of mission risks and costs, and its reviewers may be
more risk adverse and conservative than the mission proposers in assessing
likely costs and risks.

For this Discovery competition, NASA’s managers have changed the rules
in a key way that will help outer planet proposals. In previous competitions, the costs of
mission operations had to be included in the mission cap. A mission to Mars with an operations lifetime
of two to three years had an inherent advantage over an outer planets mission
that might take five to seven years to reach its target and then require
another year or two of operations. Now
NASA has excluded “reasonable” mission operations costs from the cost cap
(which means it picks up those costs separately). This goes a long way to leveling the playing
field between inner and outer solar system Discovery proposals.

In the last Discovery competition, a mission to land on a lake in the
north polar region of Saturn’s moon Titan made it to the list of
finalists. (The Mars InSight geophysical
lander was the winner.) If either ELF or
IVO is selected this time, then outer solar system will have been opened to
exploration by a new, low cost class of missions. If neither mission is selected, then the
experience learned from these proposals will become part of the community
experience that is likely to sharpen future Discovery proposals for the outer
solar system. I believe that eventually
an outer planets Discovery proposal will find the right formula for selection;
I hope that this happens sooner rather than later.

You can read the original presentations for these proposals as well as the other presentations from the OPAG meeting here.

Saturday, March 7, 2015

When I first learned about the solar system a few decades ago, the
scientific consensus held that the structure of our solar system resulted from standard
processes of stellar evolution. The
arrangement of planets with small rocky worlds closer to the star and gas and
ice giants further out would be the normal arrangement of planetary systems. Then we began to find planets around other
stars and we learned that our solar system is – if not an oddity – by no means
typical. With just one example, it is
easy to be led astray.

In our solar system, there are only two large rocky worlds, Venus and
Earth. Mercury and Mars are small enough
that both lost most of their internal heat billions of years ago and they have largely
ceased to further evolve. (The ancient,
preserved, surface of Mars is what makes it so attractive to explore for the
types of habitable environments that were long ago erased from the Earth’s
surface.) Both Venus and the Earth,
however, retain substantial heat in their cores. That heat drives plate tectonics on our world
and appears to have caused the near global resurfacing of Venus in the last few
hundred millions of years (which counts for recent when compared to the age of
the solar system).

While Venus and Earth have similar sizes and are solar system
neighbors, they have evolved very differently.
Venus today lacks oceans, appears to lack plate tectonics, and has a massive
carbon dioxide atmosphere that creates a greenhouse effect that makes the
surface a hot hell. Understanding why
Venus and Earth became so different will help us understand why Earth evolved
as it has and what the range of conditions for similarly sized worlds around
other stars may be. Venus provides the
contrast to the Earth that can help us both better understand the origins of our
world’s characteristics and the range of possibilities for similar sized
planets orbiting other stars.

Cover page for the EnVision proposal.

Today, our knowledge of Venus’ surface and its interior is similar to
our knowledge of Mars in the 1970s following the Viking mission. The
Soviet Union placed several probes on the surface that made simple measurements
in the hour or so before the surface heat fried their electronics. NASA’s Magellan spacecraft mapped the surface
with radar in the early 1990s at about 120 m resolution globally. We know, however, from our experiences
mapping the moon and Mars’ surfaces that teasing out the details of geologic
processes requires mapping surfaces with resolutions less than 50 m resolution
with smaller areas mapped at a few meters resolution.

Mapping Venus’ surface (with one exception we’ll return to later)
requires using imaging radars that can penetrate its thick cloud cover. The technology in the early 1990s when Magellan
flew was relatively new and crude by today’s standards. Now imaging radars are widely used to study
the earth both from airplanes and from satellites. The technology is mature and relatively low
cost.

As a result, something of a cottage industry has grown up proposing new
missions to map Venus either through the European Space Agency’s Medium Class
program or through NASA’s Discovery program.
The different accounting rules applied by the two agencies make direct
cost comparisons difficult, but these missions cost in the neighborhood of
$500M to $600M. A Venus radar mapping mission
has been proposed for the current ESA Medium Class competition, and I hear that
up to three missions are in competition for selection through the NASA program.

The European selection process tends to be more open than the U.S.
process, and the EnVision team led by Dr. Richard Ghail at Imperial College
London shared a copy of their proposal to ESA with me.

Unfortunately, the EnVision mission will
not move forward for the M4 competition.
From the team’s Facebook page: “ESA announced this morning
that EnVision has been evaluated as 'incompatible with the technical
and/or programmatic boundary conditions for the M4 Call'. Essentially this means that ESA believe we
would over-run on cost and/or schedule. We await further feedback which will
inform our proposal to the M5 Call expected later this year.”

While EnVision is out for this current M4 contest, reviewing its
proposal can still let us see what type of Venus mapping missions are being proposed. The proposals to NASA’s current Discovery
program will have differences from EnVision and the cost assumptions are different. Also, ESA is expected to begin the
competition for its 5th Medium Class mission later this year and
there may be a larger mission budget.
The EnVision team hopes to propose this mission, perhaps with
modifications, in the next competition.

To get a mission selected for Venus requires playing the long
game. Each competition that a team
doesn’t win gives them feedback on how to improve their proposal.

So let’s look at what a Venus mapping mission might look like using the
EnVision proposal as are guide.

The EnVision mission would address several key questions:

The average age of Venus’ surface is just a few hundred million years
old, a tiny fraction of the age of the surfaces of most rocky and icy moons in
the solar system.What processes
resurfaced the planet?Did they occur in
the same time period or have they been spread over time?

Is Venus currently geologically active and therefore continuing to
remake its surface and release new gases into the atmosphere?

What processes modify rocks once they are delivered to the
surface?Venus’ atmosphere is so thick
that its surface in many ways is similar in terms of pressure to what is found
at the bottom of our oceans.This should
lead to complex weathering and erosion, which is consistent with what we saw
from the pictures taken on the surface by the Soviet Union’s Venera landers.

What is the internal structure of Venus like?This is the part of a planet we can never
see, but scientists can study it indirectly through the combination of Venus’s
gravity field and surface topography.Both were mapped by Magellan, but at too crude of resolutions to answer
key questions.

To address these questions, the EnVision spacecraft would carry four
instruments.

These images, derived from radar imaging of Hawaii, show the improvement in resolution possible with modern radar systems compared to the data returned by the Magellan mission. This image is from a poster describing a 2012 JPL VERITAS mission that was proposed for the NASA Discovery program. Credit: JPL

The EnVision spacecraft’s primary instrument would be its VenSAR
synthetic aperture radar. Operating in
its primary mode, VenSAR would map almost the entire planet in stereo at a
resolution of 27 m. The radar would
produce both images (the equivalent of images from a camera) as well as high
resolution measurements of the absolute elevation of the surface to map its
topography. VenSAR would have a number
of special modes that would enable other forms of mapping. Small splotches of the surface would be
mapped with resolutions as fine as 1 to 2 meters, which would allow, for
example, the instrument to spot the Venera landers on the surface. An interferometric mode would enable EnVision
to spot tiny changes in relative elevation in a location that could indicate
movement from a seismic event or the swelling of a volcano. By using different polarizations of the radar
beam, the spacecraft will be able to map differences in texture across the
surface to distinguish, say, a plain covered with rocks too small to image
directly versus a plain covered in sandy material.

These figures show the improvement in topographic resolution possible with a modern radar instrument using data from Iceland. The top image shows the equivalent Magellan resolution and the bottom image shows simulated resolution possible from a new Venus radar mission. Credit: Decadal Survey White Paper, NASA

Example of the ability to detect geologic changes in vertical heights as small as a few centimeters. This example is from an Earth-orbiting radar mission for changes following a terrestrial earthquake in 1992. Credit: EnVision proposal, Fialko 2004; Peltzer et al. 1998.

Mapping mode

Resolution

Prime mission

Extended mission

Stereo imaging

27 m

91.5%

99.9%

High resolution imaging

7.3 m

0.9%

11.1%

Spotlight imaging

1.15 m

Selected locations

Selected locations

Interferometry

27 m

40.3%

40.3%

Polarimetry

63 m

6.2%

32.6%

Expected mapping coverage of Venus by the
VenSAR instrument for the proposed prime and possible extended missions. The Spotlight mode would be used for only
selected areas such as the locations of the Venera landers.

The VenSAR instrument cannot see below the very top of the surface of
Venus. EnVision would carry a low
frequency radar sounder whose beams would penetrate several hundred meters below
the surface. The result is a radargram
that looks a bit like a sonogram or x-ray of the stratigraphy of the upper
surface. Two similar instruments are at
Mars now, where they have examined the distribution of soils and ice at that
world. At Venus, this instrument would
study the depths of lava flows and sedimentary rock layers and search for
faults and folds that indicate past tectonic activity.

Example of subsurface stratigraphy revealed by a low frequency radar sounder for the Martian northern polar cap. Credit: NASA/ESA/JPL-Caltech/ASI/University of Rome/University of Washington St. Louis

The third instrument, the Venus Emission Mapper (VEM) would study Venus
in an entirely different way than the radar instruments. The Galileo and Venus Express spacecraft’s
instruments discovered narrow spectral windows where thermal emissions can be
transmitted through the otherwise opaque clouds. These few windows would give the VEM
instrument the ability to map thermal hotspots that would indicate areas of
current volcanic activity, map differences in the composition of the surface,
and detect changes in key atmospheric gases that could indicate the eruption of
a gas-spewing volcano. Because Venus’
thick atmosphere would scatter the light, the surface resolution of VEM would
be low, around 50 km. It could make,
however, ground breaking measurements of the surface variation. The recently completed Venus Express mission
carried out some measurements using this technique, but its instrument wasn’t
optimized for measurements using these spectral bands. The VEM instrument would provide much more
sensitive measurements.

The EnVision radio system would be its fourth instrument. By tracking minute differences in radio frequency
caused by the spacecraft speeding up or slowing down as the mass of the planet
below it varies, the spacecraft can study variations structure deep below the
surface.

EnVision has been proposed for ESA’s fourth Medium-class scientific
mission. The ESA competitions pit
proposals from across space science against each other, so the EnVision
proposal will be judged against both other solar system missions as well as
those that would study astrophysics and the Earth’s magnetosphere. The next key milestone will come in the next
month or two when ESA’s managers select several finalists for more detailed
analysis. If the EnVision mission is the
final selection, it would launch in December 2024 and would arrive at Venus a
few months later. The prime science
mission would take approximately two and a half years, although the team hopes
that the mission would be extended for a second 18 month observing
campaign.

The Envision team expects to hear the result of the technical review of
their proposal in a week or two (it’s a straight pass or fail). If they pass on
the technical review, the proposal then goes through a science review stage
with ESA expected to make the final selection of M4 candidates in
May/June. You can find more information
on the proposal at their website: http://www.envisionm4.net/

When I read about proposals for most small-scale planetary missions
(ESA’s Medium class, NASA’s Discovery class) their goals are often narrowly
focused so that they can be met within a constrained budget. In the last Discovery mission selection,
several Venus mapping missions were proposed.
I’ve heard that each focused its science questions in different somewhat
narrow ways that lead review panels to be confused about what the actual
priority would be. For me, the true
value of a mission like EnVision is the breadth of its data. Scientists could mine the data from a mission
like this for many years and use the data to develop and then answer questions
that we don’t yet know enough to ask.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.